BMW 6 Series E63 - Belt Replacement / Tensioner

[Suresurf]

Whilst servicing my car I thought I would replace the belts. The aircon belt has no tensioner. How do you replace?

The large serpentine belt has a tensioner but unlike my previous 5 Series, it does not appear to be spring loaded. When I put a wrench on it, the bolt started to undo! How do you tension these belts. I stopped before it loosened so both old belts are still on the car.

Thanks,